Composite structures often are fabricated in a laminated array, typically including a core, such as of foam, sandwiched between a pair of outer structural skins, typically fabricated of a resin and a reinforcing fiber, and bonded together by some sort of adhesive. Such composite structures typically are generally planar in geometry due to limits in processing technology. Thermoset resin systems predominantly are used with such composite structures. Epoxy resins, bismaleimeide, and polyimide resins are typical thermoset resins which are used. They have short shelf lives, require refrigeration and careful handling and storage. The reinforcing fibers typically have a tensile modulus much greater than the resin, and are much stronger. The composite material resulting from their combination thereby is enhanced. Typical reinforcing fibers are graphite, glass and Kevlar. The cores typically are fabricated of foam or honeycomb construction. For instance, cores can be made of lightweight, low-density materials, such as polymethacrylimide foams, phenolic foams, aramid honeycomb, aluminum honeycomb, and the like. Composite structures are made from these materials by machining the core and then bonding the composite material skin to the core. Such composite structures have found widespread acceptance in various industries, such as the aerospace industry, because they can save significant amounts of weight over previously used metallic parts.
Due to labor intensity in processing composite structures of the character described, particularly those structures fabricated from thermosetting materials, considerable expense is involved, as well as time-consuming preparation of the core. They also are not readily repairable. Fully cured structures offer few free polymer chains for molecular level bonding to a repair material. Techniques have been devised to foam-in-place an epoxy core between the skins of composite material. However, the interface is weak because of the required adhesive between the parts.
On the other hand, thermoplastics are reformable, readily repairable and inherently much more impact-resistant than thermosets. The use of thermoplastics for continuous fiber reinforced structures has, however, been limited by an inability to achieve consolidated structures with suitable dispersion of resin, fiber reinforcements and other components. Nevertheless, improvements are being made. For instance, U.S. Pat. No. 4,837,251, dated June 6, 1989, assigned to the assignee of this invention, and which is incorporated herein by reference, a new and improved composition for a pressure molded core of a composite structure is disclosed. In addition, a new and improved method of fabricating a composite structure is shown in copending application Ser. No. 139,007, filed Dec. 29, 1987, (Docket No. B02676). As disclosed therein, a skin composed of reinforcing material and a thermoplastic matrix is provided. The skin is thermoformed to the general exterior shape of the composite structure. A core is molded into the general shape of the composite structure, the core having a thermoplastic matrix compatible to the matrix of the skin. The composite structure then is final-formed by placing the skin and the core in juxtaposition in a forming mold, and a thermoplastic coating material is injected into the mold under isostatic conditions. The thermoplastic coating material is compatible to the thermoplastic matrices of the skin and the core. Therefore, the thermoplastic matrices of the skin, the core and the coating molecularly bond the entire composite structure into an integral, substantially homogeneous construction with the molecular bond about and throughout the parts of the composite structure.
In U.S. Pat. No. 4,013,810 to Long, dated Mar. 22, 1977, thermoplastic materials are used to form the skins of a sandwich structure along with a core of a compatible thermoplastic material and a filler of hollow glass spheres. However, the core is preformed or cast into an initial shape and then the face sheets or skins are manufactured to the shape required and placed over the foam core and remolded under pressure and heat.
Lastly, an example of an advanced composite structure with which the invention is readily applicable is shown in copending application Ser. No. 191,250, filed May 6, 1988, (Docket No. B02490), assigned to the assignee of this invention, and which is incorporated herein by reference. That application involves an irregularly shaped housing, such as for a dynamoelectric machine, and includes various components such as conduits, electrical devices, etc. embedded within the core of the composite structure.
This invention is directed to a further improvement in fabricating composite structures to provide a molecular bond between a core and one or more skins of the composite structure.